- Create Functions Using Arrow Syntax 4:34
- Concise Arrow Function Syntax
- Arrow Functions Review 5 questions
- Default Function Parameters 5:11
- Create an Arrow Function 1 objective
- Descriptive Comments for Functions
- Function Challenge 1:24
- Function Challenge Solution 4:28
- Testing for Number Arguments 4:02
- JavaScript Functions Review 6 questions
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Testing for Number Arguments
4:02Testing for Number Arguments
Now we need to make sure that
the getRandomNumber function
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receives two numeric values as arguments.
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And no other type of values.
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If the function is called
with a string for example,
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then the program should display
a custom error message.
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Inside the function add an if statement
just above the return statement.
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But what should we test here?
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Well, we need to make sure that the two
arguments passed to the function
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are numbers.
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There's a few ways we might do this.
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I’ll teach you a new method for
testing number values.
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JavaScript provides a special function
called isNaN, or is not number.
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That takes one argument and
returns a Boolean value.
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It returns true if the value is
not a number, and false if it is.
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For example, if you pass the string 'six'
to the function, you get the value true.
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However if you pass a number, like six,
to the function, you get false.
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So in the if statement,
I'll first test that the lower argument
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passed to the function is a number,
like this.
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In other words,
if the lower value is not a number,
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then the code block with
this if statement will run.
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But we want to test both arguments.
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If either is not a number,
then we'll display an error.
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To test both you can use the logical or
operator, by typing two pipe characters.
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Then add a test for
the upper argument like this.
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So now if either argument is not a number,
we can display a custom error.
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We could maybe use console.log to
display a message in the console.
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Like 'Both arguments must be numbers.'.
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In this case, it's probably
best to throw a custom error.
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So the programmer can immediately see that
they've used the function incorrectly.
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JavaScript lets you create your own errors
which print directly to the JavaScript
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console, just like
regular syntax errors do.
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It looks like this.
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The throw keyword,
followed by JavaScript's Error object.
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This is a built in keyword and
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object JavaScript provides
to create a custom error.
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Or, as developers say, throw an exception.
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Since it's inside the if statement,
the error is thrown
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only when the function receives anything
but a number for either argument.
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All right, now I'll test the latest
changes by calling the function correctly
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with numbers a couple of times.
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Then call it with a string
value like 'three hundred'.
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I'll save the file.
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And over in the console,
I get two random numbers, 4 and 17.
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And below them is the Uncaught Error,
displaying my custom error message,
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Both arguments must be numbers.
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Notice how it looks just like a regular
error thrown by the JavaScript engine.
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It even displays the line number
producing the error, great.
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So now you've learned how
to program defensively.
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Or looking out for problems that might
arise in your program and taking action.
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As with many things in programming,
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getting comfortable with the details
of functions takes time and practice.
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But once you get to hang of working with
functions, you'll be well on your way to
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mastering one the most essential
features of JavaScript.
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In fact, functions are often referred to
as first-class citizens in JavaScript.
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Which means that you have the ability
to do almost anything with functions.
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Like store them in variables,
pass them as arguments to other functions.
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You can even set a function
to return another function.
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This lets you take advantage of powerful
design patterns you'll learn about in
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later courses.
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So you should feel excited and
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proud of reaching another important
milestone in your programming journey.
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And remember, we're here to help.
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So if you have questions about anything
covered in this course, you can always get
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in touch with the friendly Treehouse
staff or other students in the community.
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Thanks everyone, and happy coding.
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